The present technology relates to a solid state imaging device, a driving method of a solid state imaging device, and an electronic device, and more particularly to a solid state imaging device, a driving method of a solid state imaging device, and an electronic device, which are capable of suppressing a variation in a signal of high illuminance exceeding a saturation level of a solid state imaging device.
In general metal oxide semiconductor (MOS)-type image sensors in which charges generated and accumulated in a photoelectric conversion unit (photo diode (PD) according to an incident light quantity are read through a MOS transistor, a saturation level thereof is restricted according to a charge amount chargeable in the photoelectric conversion unit. In other words, it is difficult to properly detect a light quantity of a range exceeding the saturation level of the photoelectric conversion unit.
Therefore, for example, when the diaphragm or the shutter speed is adjusted to a dark portion of a subject, it is difficult to obtain an image for a bright portion of the subject due to saturation of the photoelectric conversion unit. On the other hand, when the diaphragm or the shutter speed is adjusted to the bright portion of the subject, since charges are not sufficiently accumulated, it is difficult to obtain an image for the dark portion of the subject, or an image quality degrades since a signal to noise (S/N) ratio is not sufficiently obtained.
In order to solve the above problem, a technique of increasing a dynamic range by capturing an image in a short period of time so that an amount of charges accumulated in the photoelectric conversion unit is not saturated while changing the shutter speed, that is, an exposure time, in the photoelectric conversion unit, capturing an image in a sufficiently long period of time so that charges can be accumulated even at darkness at which an amount of charges is not accumulated during a short period of time, and then combining the images is known.
However, in this technique, since a frame memory is necessary, the size of a device increases, and the cost increases. Further, since two signals that differ in exposure period of time are combined, it is difficult to apply the technique to a moving subject.
A technique in which neighboring rows in a pixel region are different in the exposure period of time and thus a memory is unnecessary is also known. However, in this technique, since a single signal is generated using two pixels, the resolution deteriorates.
In this regard, a technique in which both a channel voltage of a charge transfer unit connected to a photoelectric conversion unit and a charge-voltage conversion unit (floating diffusion (FD)) and a channel voltage of a charge reset unit whose one end is connected to the charge-voltage conversion unit and whose other end is connected to a predetermined voltage have a polarity different from a polarity causing a conduction state has been proposed (see JP 3827145 B (JP 2003-18471 A).
In this technique, charges are caused to overflow from the photoelectric conversion unit to the charge-voltage conversion unit and then from the charge-voltage conversion unit to a predetermined power source, a voltage of the charge-voltage conversion unit at that time is used as a first light detection voltage, and a voltage of the charge-voltage conversion unit after charges of the charge-voltage conversion unit are reset by the charge reset unit and charges accumulated in the photoelectric conversion unit are transferred by the charge transfer unit is used as a second light detection voltage.
Here, the first light detection voltage is a signal corresponding to the logarithm of illuminance, and the second light detection voltage is a low-noise signal accumulated in the photoelectric conversion unit. Thus, the two signals can be used as necessary such that the low-noise signal accumulated in the photoelectric conversion unit is used in the case of low illuminance, and a signal having a large dynamic range corresponding to the logarithm of illuminance is used in the case of high illuminance.
Further, a technique in which an intermediate potential is applied to the charge transfer unit so that charges overflow from the photoelectric conversion unit to the charge-voltage conversion unit, a highly sensitive linear signal is used in the case of low illuminance, and a signal of a large dynamic range corresponding to the logarithm of illuminance is obtained when there is much highly illuminant incident light has been proposed (see JP 2006-303768 A). In the following, a signal corresponding to the logarithm of illuminance is referred to as logarithmic compression of a signal corresponding to illuminance.